The invention relates to the general field of CPP GMR read heads with particular reference to increasing transverse resistance without sacrificing dimensional control.
The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are seed layer 11 on which is antiferromagnetic layer 12 whose purpose is to act as a pinning layer for magnetically pinned layer 345. The later may be a single ferromagnetic layer or, preferably, a synthetic antiferromagnetic (formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 and 15). This results in an increase in the size of the pinning field so that a more so pined layer is obtained. Next is non-magnetic spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. A contacting layer 18 lies atop free layer 17 and cap layer 19 is present over layer 18 to protect the structure during processing.
When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
Most GMR devices have been designed so as to measure the resistance of the free layer for current flowing parallel to the film""s plane. However, as the quest for ever greater densities continues, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack, so these layers should have resistivities as high as possible while the resistance of the leads into and out of the device need not be particularly low. By contrast, in a CPP device, the resistivity of the leads tend to dominate and should be as low as possible.
It can be shown that the greater the Resistancexc3x97Area (RA) the greater the sensitivity of the device, i.e. a device having both high resistance as well as high cross-sectional area is to be desired. At first glance these appear to be conflicting requirements since increasing the area decreases the resistance. To overcome this problem, several groups have proposed to increase RA by inserting a NOL (nano-oxide layer) within a CPP GMR structure [1]. The idea is to reduce the available area through which current can flow while continuing to maintain the physical dimension of the reader head relatively large. Fujitsu [1] used a layer of CoFeB oxide at the outer interface of the free layer to increase RA by forcing current to flow through pinholes in the CoFeB oxide layer. This increased the RA to between about 0.25 and 1 ohm.xcexcm2. Toshiba [2] used a NOL of aluminum oxide over copper in the spacer layer to confine current to passing through unoxidized Cu. This increased RA to between about 0.25 and 0.7 ohm.xcexcm2.
The main problem with the NOL process described above is that it relies on the appearance of pin holes within a film that, under ideal conditions, should have none. This makes for a non-reproducible process giving unpredictable results. In this disclosure, a different concept and method of making the NOL layer in a CPP GMR are disclosed.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,452,763, Gill discloses a GMR design with a nano-oxide layer in the second anti-parallel pinned layer. U.S. Pat. No. 5,715,121 (Sakakima et al.) shows a related process for a MR element. In U.S. Pat. No. 5,627,704, Lederman et al. show a GMR CPP transducer with a flux guide yoke structure while in U.S. Pat. No. 5,668,688, Dyker et al. show a CPP GMR structure.
The two references numbered in the preceding text are.
[1] Fujitsu abstract Intermag Europe 2002 Amsterdam.
[2] Toshiba abstract Intermag Europe 2002 Amsterdam.
It has been an object of at least one embodiment of the present invention to provide a process to manufacture a CPP GMR magnetic read head.
Another object of at least one embodiment of the present invention has been that devices resulting from application of said process have a high resistancexc3x97area product.
Still another object of at least one embodiment of the present invention has been that said process be compatible with existing processes for the manufacture of CPP GMR devices.
These objects have been achieved by increasing the transverse resistance of one or both of the non-magnetic conductive layers (i.e, the spacer and the contacting layers) without reducing their overall area. To accomplish this, a NOL (nano-oxide layer) is inserted through the middle of the conductive layer in question. A key feature of the invention is that the NOL is formed by first depositing the conductive layer to about half its normal thickness. Then a metallic film is deposited thereon to a thickness that is low enough for it to still be discontinuous i.e. to consist of individual islands. These islands are of a material that can be fully oxidized without significantly oxidizing the conductive layer on which they lie. This substructure is competed by depositing the remainder of the conductive layer to a thickness sufficient to fully enclose the islands of oxide. The process can be applied to the spacer, the contacting layer, or both. In all cases a significant increase of the resistance-area product of the device is obtained.